The antimicrobial quantitative structure–activity relationship of plant flavonoids against Gram-positive bacteria was established in our previous works, and the cell membrane was confirmed as a major site of action. To investigate whether plant flavonoids have similar antibacterial effects and mechanisms against both Gram-negative and Gram-positive bacteria, here, the minimum inhibitory concentrations (MICs) of 37 plant flavonoids against Escherichia coli were determined using the microdilution broth method, and then the correlation between their lipophilic parameter ACD/LogP or LogD_7.40 value and their MIC was analyzed. Simultaneously, the correlation between the ACD/LogP or LogD_7.40 value and the MIC of 46 plant flavonoids reported in literature against E. coli was also analyzed. Both sets of results showed that there is a significant correlation between the LogP value and the MIC of plant flavonoids against Gram-negative bacteria. However, it is difficult to effectively predict the MIC of plant flavonoids against Gram-negative bacteria from their lipophilic parameters. By comparing two regression curves derived from plant flavonoids against Gram-negative and Gram-positive bacteria, it was further discovered that the antibacterial activities of most plant flavonoids against Gram-negative bacteria are stronger than those against Gram-positive bacteria when their LogP values are less than about 3.0, but the opposite is true when their LogP values are more than about 3.6. Moreover, this comparison also suggests that unlike mainly acting on the cell membrane of Gram-positive bacteria, plant flavonoids have multiple mechanisms against Gram-negative species, while the cell membrane is also an important action site among them. Combined with the correlation analyses between the enzyme inhibitory activity and the LogP value of the reported flavonoids, it was further suggested that DNA gyrase is another important target of plant flavonoids against Gram-negative bacteria.

Four Chinese herbs Zhishi, Zhiqiao, Qingpi and Chenpi from Citrus genus were widely used for treating various cardiovascular and gastrointestinal diseases. Although many pharmacological functions of these herb decoctions can be clarified from the bioactivities of identified ingredients, some of them remain confusing. To clarify the reasons of those confusing functions, depending on the extended structure-activity relationships of cholinergic and anti-cholinergic agents, here a simple method for quickly discovering possible choline analogs was established using a specific TLC method, and then stachydrine and choline were first identified from these Citrus herb decoctions based on their NMR and HRMS data. After this, two TLCS methods were first established respectively for the quantitatively analyses of stachydrine and choline, and the contents of both two ingredients and synephrine in 39 samples were determined using the valid TLCS and HPLC methods, respectively. The results showed the contents of stachydrine were 2.4 times to those of synephrine in Zhiqiao while about 1/3 to 2/3 in Zhishi, Qingpi and Chenpi. Simultaneously, the contents of stachydrine, choline and synephrine in these herbs present similar changing trends along with the delay of harvest time. However, the contents of synephrine decrease most rapidly, while those of stachydrine decrease most slowly. Based on these above, comparing with the pharmacological activities and pharmacokinetics reported of stachydrine and synephrine, it indicated that stachydrine and synephrine can be considered as a pair of bioactive equilibrists, especially in the cardio-cerebrovascular protection from these citrus herbs. Simultaneously, it confirmed that stachydrine should play an important role in the pharmacological functions, especially in dual-directionally regulating the uterus and various beneficial effects on cardio-cerebrovascular system, kidney and liver, of these citrus herbs.

Antimicrobial resistance (AMR) has been a serious threat to human health, and combination therapy is proved to be an economic and effective strategy to fight the resistance. However, the abuse of drug combinations would conversely accelerate the spread of AMR. In our previous work, it had been concluded that the mutant selection indexes (SIs) of one agent against a specific bacterial strain are closely related to the proportions of two agents in a drug combination. To discover probable correlations, predictors and laws for further proposing feasible principles and schemes guiding the AMR-preventing practice, here three aspects were further explored. First, the power function (y=ax^b, a > 0) correlation between the SI (y) of one agent and the ratio value (x) of two agents in a drug combination was further established based on the mathematical and statistical analyses for those experimental data, and two rules a₁ × MIC₁ = a₂ × MIC₂ and b₁ + b₂ = -1 were discovered from both equations of y=a₁x^b1 and y=a₂x^b2 respectively for two agents in drug combinations. Simultaneously, it was found that one agent with larger MPC alone for drug combinations show greater potency for narrowing itself MSW and preventing the resistance. Second, a new concept as mutation-preventing selection index (MPSI) was proposed and used for evaluating the mutation-preventing potency difference of two agents in drug combinations, and the positive correlation between the MPSI and the mutant prevention concentration (MPC) or minimal inhibitory concentration (MIC) was subsequently established. Inspired by this, the significantly positive correlation, contrary to previous reports, between the MIC and the corresponding MPC of antimicrobial agents against pathogenic bacteria was established using one hundred and eighty-one of data pairs reported. These results together of above three aspects indicate that the MPCs in alone and combination are very important indexes for drug combinations to predict the mutation-preventing effects and the trajectories of collateral sensitivity, and while the MPC of an agent can be roughly calculated from its corresponding MIC. Subsequently, the former conclusion was further verified and improved by the antibiotic exposure to forty-three groups designed as different drug concentrations and various proportions. The results further proposed that the C/MPC for the agent with larger proportion in drug combinations can be considered as a predictor and is the key to judge whether the resistance and the collateral sensitivity occur to two agents. Based on these above correlations, laws, and their verification experiments, some principles were proposed, and a diagram of the mutation-preventing effects and the resistant trajectories for drug combinations with different concentrations and ratios of two agents was presented. Simultaneously, the reciprocal of MPC alone (1/MPC), proposed as the stress factors of two agents in drug combinations, together with their SI in combination, is the key to predict the mutation-preventing potency and control the trajectories of collateral sensitivity. Finally, a preliminary scheme for antimicrobial combinations preventing the AMR was further proposed for subsequent improvement research and clinic popularization, based on the above analyses and discussion. Moreover, some similar conclusions were speculated for triple or multiple drug combinations.

Plant flavonoids have increasingly paid a close attention to for new antimicrobial agents or adjuvants. In our previous work, it was confirmed that the cell membrane is the major site of plant flavonoids acting on the gram-positive bacteria, and which likely involves the inhibition of the respiratory chain. Inspired by the similar structural and antioxidant characters of plant flavonoids to MKH₂, we deduced that the quinone pool is probably a key target of plant flavonoids inhibiting gram-positive bacteria. To verify this, twelve plant flavonoids with six structural subtypes were preliminary selected, and their MICs against gram-positive bacteria were predicted from the antimicrobial quantitative relationship of plant flavonoids to gram-positive bacteria. The results showed they have different antimicrobial activities. After their MICs against S. aureus were determined using broth microdilution method, nine compounds with the MICs ranged from 2 to 4,096 μg/mL or more than 1,024 μg/mL were eventually selected, and then their MICs against S. aureus were determined interfered with different concentrations of MK-4 and the MKs extracted from S. aureus. The results showed that the greater the antibacterial activities of plant flavonoids were, the more greatly their antibacterial activities decreased along with the increase of the interfering concentrations of MK-4 (from 2 to 256 μg/mL) and MK extract (from 4 to 512 μg/mL), and while those, with the MICs equal to or more than 512 μg/mL, decreased a little or remained unchanged. Especially, under the interference of MK-4 (256 μg/mL) and MK extract (512 μg/mL), the MICs of α-mangostin, a compound with greatest inhibitory activity to S. aureus in these twelve plant flavonoids, increased by 16 times and 8 to16 times, respectively. Based on these above, it was proposed that the quinone pool is a key target of plant flavonoids inhibiting gram-positive bacteria, and which likely involves multiple mechanisms including some enzyme and non-enzyme inhibitions.